Process for obtaining a composite material and composite material obtained by said process

ABSTRACT

A process for producing a composite material comprising a base having a nickel surface and a gold layer metallurgically bonded to the nickel of the base and having sufficient nickel to increase the hardness of the gold layer but insufficient to significantly destroy the distinctive color of the gold layer comprises depositing the gold layer on the nickel and annealing the composite structure to provide the metallurgical bond between the nickel and gold and to interdiffuse said metals, and the composite structure obtained by said process. The composite material is particularly useful as a coinage structure.

The present invention is concerned with a composite materialparticularly useful for tokens and coins, and more particularly withtokens or coins having a nickel base and a gold surface.

Essentially pure or low alloy content gold coins have been used sincethe time of the Lydians but in today's world, such coins must have highface value because of the intrinsic value of the metal. As of thiswriting gold has an intrinsic value of the order of $15.60 (Canadian)per gram. As of now, gold coins exist only essentially as collector orinvestment items and do not circulate in the sense of that term asapplied to 25¢ pieces. If widespread circulation of gold coins existed,it would be found that gold, like silver, is too soft for everydaycoinage use.

For these reasons the Canadian 25¢ piece was changed from asilver-copper alloy to pure nickel in 1968. In 1965 the U.S. 25¢ piecewas changed from a 90 silver-10 copper alloy to a 75% Cu-25% Ni facelayer sandwiching a pure copper core. Other countries have switched tocopper based coinage containing Cu plus Al or Zn or Ni or a combinationof the latter elements.

Actual studies of the wear of coinage alloys as a function of the numberof years in service are relatively few, but the wear rates for theAg-Cu, Cu-Ni and Ni are well established and are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        HISTORICAL WEAR DATA                                                                                       Wt.   Average Wear                               Year   Coin     Material     (g)   Rate mg/yr                                 ______________________________________                                        Current                                                                              U.S. 5¢                                                                           75% Cu--25% Ni                                                                             5.0   4                                          Pre-1965                                                                             U.S. 25¢                                                                          Ag--Cu*      6.25  7                                          Pre-1968                                                                             Can. 25¢                                                                          Ag--Cu*      5.85  6                                          Current                                                                              Can. 5¢                                                                           Ni*          4.55  0.9                                        ______________________________________                                         *Canada's New Nickel Coins, reprinted from Canadian Mining Journal,           December, 1968.                                                          

Examination of Table 1 shows that the best alloy from a wear viewpointis pure Ni whose wear rate is 1/4 that of cupro-nickel (75% Cu, 25% Ni)and 1/6 that of the 90% Ag-10% Cu formerly used. Nickel is also superiorto Cu-Ni alloys and Ag-Cu alloys in terms of corrosion properties.Nickel maintains its bright appearance for longer than its normalservice life of 20 to 40 years. The next best alloy as far as corrosionproperties of existing coinage is cupro nickel (75% Cu-25% Ni).

Thus, for a silver colored coinage material pure nickel is by far thebest. It also possesses unique magnetic properties between those of ironand non-magnetic alloys which allow it to be easily discriminated ineither mechanical or electrical coin operated vending machines.

Automated vending machines, telephones, transit services, etc. arerapidly becoming more prevalent. For instance the number of vendingmachines in Canada is now estimated to exceed one million units. In manycases the object vended exceeds the value of one dollar and requires anexcessive number of 25¢ coins. There is a large need for coins of facevalue exceeding 25¢. In addition to requirements for high denominationcoinage for vending machines there is an advantage to saving costsassociated with circulating new paper money, e.g. in excess of 5¢ peryear to replace a paper dollar.

However, if one, two and perhaps five dollar coins were added to theexisting 5¢, 10¢, 25¢ coins a problem would arise. If the coins were allpure nickel and the size gradually increasing, the size of the dollarcoins would be too large to carry around in a pocket. If the sizes wereintermediate between 5¢, 10¢ and 25¢ and all the coins were pure nickelthe public would likely find coin discrimination difficult. If the coinswere made of a brass, bronze, or cupro nickel bronze alloy they wouldlikely have inadequate wear properties and they would also tarnishquickly in service.

Cupro-nickel alloys (75% Cu-25% Ni) have the best corrosion or tarnishresistance of all of the copper base alloys, but they have a silvercolour and the level of nickel must be reduced to less than 10% in orderto obtain a golden color. At this level of nickel the alloy tarnishes.Attempts to introduce other elements such as Al, Zn into thecupro-nickel alloys have yielded golden-colored alloys of onlymarginally improved tarnish resistance, none being as good ascupro-nickel, and all being very inferior to pure nickel.

If a brass, bronze or other cupro nickel bronze alloys are employed forhigh denomination coins and they perform in an inferior fashion bothphysically and in appearance to the pure existing low denominationnickel coins, the high denomination new coins will likely lack publicacceptance.

STATEMENT OF PROBLEM

A new material is required which has wear properties as good as or closeto pure nickel, has the same tarnish resistance as pure nickel, and hasa gold color.

BRIEF DESCRIPTION OF DRAWING

The accompanying FIGURE is a graph in which accelerated coinage wear(weight loss of coins in mg) is plotted against time in test (days).

DESCRIPTION OF THE INVENTION

The present invention contemplates a composite material, particularly acoin or token structure, comprising a base having at least a surface ofnickel and bearing on the surface thereof a layer of gold at least about0.1 μm thick up to e.g. about 1.5 μm or even 2.5 μm thick. The goldlayer is metallurgically bonded to the nickel of the base and containsan amount of nickel sufficient to increase the hardness of the goldlayer but insufficient to significantly destroy the distinctive color ofthe gold layer.

The token or coin structure of the present invention can be made bydepositing, e.g. by electroplating, a layer of gold onto a nickelsurfaced base and thereafter annealing the composite structure toprovide a metallurgical bond therebetween and to interdiffuse nickel ofthe base and gold of the surface layer.

DETAILS OF THE INVENTION

In order to provide those skilled in the art with greater detail withrespect to the present invention, the following paragraphs set forthparticulars and alternatives contemplated by the inventors to be withinthe scope of the invention as hereinbefore described.

The base of the composite structure of the invention is advantageouslymade of essentially pure nickel but can be made of a high nickel alloy,i.e., containing greater than about 50% nickel, which has corrosionresistance and workability substantially equivalent to that of purenickel. However, nickel alloys containing less than 50% nickel, e.g.,75% Cu, 25% Ni, may be used but such alloys are less preferable. Whenused in this specification and claims the term "nickel" includes suchnickel alloys. Generally speaking, the base of a coin or token structureis monolithic, i.e. made wholly of a single material. However, ifdesired, the base can be a composite or a sandwich structure providedthe surface is nickel.

In physical form, the base of the token or coin structure is of cointhickness e.g. up to about 0.6 cm but can be of any desired width andlength. For example, the base can be sheet or strip out of which coin ortoken blanks can be stamped. Alternatively, the base can be a coin ortoken blank stamped from a sheet of nickel or any intermediate form ofthe coin or token. Specifically, the base can be a blank which has beensized or sized and coined, or sized, coined and rimmed or sized, coined,rimmed and milled. Those skilled in the art will appreciate that goldplating on each of the alternative types of coin or token bases willresult in the front and rear of the coin or token having a gold color.However, plating done prior to rimming and milling will result in coinshaving an edge with the appearance of nickel. Alternatively, thesubstrate can be placed on a moving band or can be masked so that thegold deposit can occur on one side or selected areas.

The gold on the nickel surface of the base is advantageouslyelectrodeposited as pure (24 KT) gold from an electroplating bath usingconditions applicable to obtaining a pure electrodeposit. For purposesof this specification and claims, however, gold need not be 100% pure.For purposes of this invention, the term "gold" includes not only puregold but also yellow alloys and reddish yellow alloys which may containsilver, copper, nickel, a platinum-group metal and combinations thereof.Gold containing up to about 8% to 10% by weight nickel will retain itsgold color equivalent to the colour of 14 KT to 18 KT gold-silver-copperalloy.

The gold layer on the nickel surface of the base is advantageouslyelectrodeposited from a cyanide type bath. Such baths are usually ofproprietary nature. However, the general types of cyanide baths andconditions of operation are set forth in standard reference sources suchas Electroplating Engineering Handbook (3 ed.) A. Kenneth Graham, VanNostrand Reinhold Company (C) 1971 page 242 and in References 1, 2, 29,30, 31, 32 and 33 listed on page 255 of that work. It is also within thecontemplation of the present invention to provide gold layers of therequisite thickness by means other than electroplating provided, ofcourse, that the quality of the adhesion of the gold layer to the nickelis at least equivalent to that provided by electroplating. As thoseskilled in the art are aware, the quality of adhesion of an electroplatedepends to a large extent on the care taken in surface preparation andcleaning of the base to be plated. In this regard Chapter 3 of theaforecited Electroplating Engineering Handbook entitled Metal SurfacePreparation and Cleaning is recommended to those desiring to practicethe present invention.

In producing the composite structure of the present invention, theannealing step to achieve the metallurgical bond between gold and nickeland to harden the gold is an important feature. Annealing can be carriedout any time after deposition of the gold layer, either before or afterworking or mechanical operation on the composite material. Theconditions of temperature and time of annealing are selected so that aninterdiffused layer of gold and nickel is achieved without loss of thedesired surface color. For example, with a pure nickel base and a puregold deposit averaging 0.3 μm thick, effective annealing in hydrogen at450° C. may be achieved in a short time e.g., in the range of about 2 to5 minutes. This annealing time has been determined as residence time ina 5 cm tube furnace under laboratory conditions. Those skilled in theart will appreciate that commercial scale furnacing may vary intemperature and time. Nickel has a significant whitening effect on gold.After approximately 6 minutes of annealing under those specifiedlaboratory conditions, the whitening effect of nickel predominates andthe golden colour is lost. Annealing under the same conditions for lessthan two minutes is insufficient to produce the required metallurgicalbond. While this example of annealing teaches an effective annealingprocedure operable in the context of the present invention, variationsare obviously possible. For example, annealing can be conducted at lowertemperatures e.g., down to about 350° C. for longer times. Hydrogen canbe replaced as an annealing atmosphere with any atmosphere, e.g.,cracked ammonia, argon, etc. which will prevent oxidation of the nickel.In addition, if alloy gold is used as the layer, less nickel diffusioncan be tolerated and therefore annealing should be carried out attemperatures lower than 450° and for times less than those employed forpure gold. Still further, if gold is employed in thicker layers e.g.,from 0.5 to 1.0 μm, somewhat greater times or higher temperatures shouldbe employed in annealing in order to permit diffusion of some nickel tothe outer-most volume of the gold layer in order to induce increasedhardness throughout the gold layer. Conversely, extremely thin goldlayers e.g., about 0.1 μm must be annealed for very short times in orderto retain the visual yellow effect of the gold. Annealing time is alsodependent upon the thickness of the nickel-containing base, generallylonger times being necessary with thicker bases. The type of heatingequipment also is determinative of annealing times. For example, withhigh frequency induction skin heating, shorter times can be used becausethere is no need to allow time for the entire base to come to annealingtemperature. In general, annealing can be carried out at temperatures ofabout 350° to about 650° C. for about thirty minutes to about a fewseconds.

The composite structure of the present invention is particularly adaptedto be used as coins or tokens, other structures in which a goldenappearance, wear resistance and corrosion reistance are desirable arewithin the contemplation of the invention. Such other structures includeflatware, plumbing fixtures and the like.

EXAMPLE 1

The following coins were dipped into a 5 wt.% NaCl solution which hadbeen adjusted to pH 4 with nitric acid once per day for four days andallowed to air dry at room temperature the balance of the day.

1. Pure Ni Canadian 25¢.

2. A U.S. 25¢ piece containing outer faces of 75%Cu-25%Ni and a coppercore.

3. A gold coated nickel coin, with the gold coating prepared accordingto the procedure detailed below.

4. A 1983 British Pound containing 75%Cu-18%Zn-7%Ni.

5. A gold coated 75%Cu-25%Zn coin, with the gold coating preparedfollowing the same procedure as 3.

Results

The surfaces of the pure Ni coin (1) and the new gold on Ni coin (3)were not visibly tarnished. The U.S. cupro-nickel 25¢ piece (2) lost itsmetallic luster but had no visible evidence of a tarnish. All the othercoins including the British Pound (4) and the Au on brass composite (5)lost their metallic luster and had objectionable tarnish formed on theirsurfaces.

These results show that the latter two coins (4) and (5) are unsuitableas high denomination coins.

Method of Preparation of the New Au on Ni Composite Coinage Material

A mint Canadian 25¢ pure nickel coin 1 was cleaned in a mild acidicsolution and plated at 60° C. at 5 milliamps per square centimeter for1-2 minutes from a standard cyanide-type Au plating bath obtained fromJohnson Matthey and Mallory Ltd. under the trademark "Orosene 999 24 KTbright gold". The weight of the Au plated was 6 mg and the apparentcalculated thickness of the Au layer was 0.3 μm. This coin wassubsequently annealed at 450° C. for 3 to 4 minutes in pure hydrogen.This procedure on this substrate gives a coin which is gold in colorlighter than the colour of 24 KT gold.

EXAMPLE 2

Eight Canadian 25¢ pure nickel coins were plated with 6 mg of Au eachaccording to the procedure in Example 1. Four of the coins weresubsequently annealed at 450° C. for 4 min in pure hydrogen, the otherfour coins were unannealed.

The thus prepared coins were subjected to wear testing. The apparatusfor wear testing in this Example and in other Examples herein comprisesa ceramic jar mill having internal dimensions of 4 inches high (10.16cm) and 5 inches wide (12.7 cm). The mill is closely lined with azippered bag made of metallographic polishing cloth. The coins areintroduced into the bag along with an equal weight of metal shot 0.3 to1.9 cm in diameter made of an alloy known as HASTELLOY™ alloy Ccontaining nominally (in weight percent) 54% nickel, 17% molybdenum, 15%chromium, 5% iron and 4% tungsten, 13 grams of leather strip about 3.8cm×0.3 cm×0.15 cm and 7 grams of cork No. 00 about 0.12 cm in diameter.The leather strip is soaked with synthetic sweat solution (40 g NaCl, 5g Na₂ HPO₄, 4 ml lactic acid, balance, to 4 l, distilled water), the bagis zippered closed, fitted into the ceramic jar mill and then rotated atless than critical speed for numbers of days with the leather beingperiodically rewet with synthetic sweat solution. The mill was rotatedfor 5 days at 45 rpm.

Results

The mean weight loss of the annealed coins was 6.8 mg while that for theunannealed coins was 9.0 mg. The annealed samples had a more uniformcolor than the unannealed sample.

EXAMPLE 3

Five Canadian pure Ni 25¢ pieces were plated with 24 KT Au, with the Auplated according to the procedure set forth in Example 1. The coins wereplaced into a 5 cm tube furnace having a H₂ atmosphere at 450° C. fortimes ranging from about 1 to about 10 min. The surface of the Au platedcoins retained their 24 KT appearance up to 3 min. After about 4 minutesthe color matched the color of 18 KT (75% Au, 15% Ag, 10% Cu) gold.After about 6 minutes the gold color had largely disappeared.

EXAMPLE 4

Two samples of the Au coated pure Ni Canadian 25¢ pieces containingabout 6 mg of Au in a layer 0.3 μm thick annealed at 450° C. for 4minutes along with two U.S. cupro-nickel 25¢ pieces and two Canadianpure nickel 25¢ pieces were tumbled in the ceramic mill at 45 rpm in acloth bag containing cork and leather wetted with a synthetic sweatsolution and an equal weight of HASTELLOY C shot as described in Example2. The weight loss of each coin was measured each day and the resultsplotted in FIG. 1. The data in FIG. 1 show that the wear rate of theU.S. cupro-nickel is approximately twice that of pure nickel and thatthe weight loss of the Au plated and annealed nickel coins wasindistinguishable from pure nickel Canadian 25¢ pieces. It was notanticipated that a coin having roughly 6 mg of gold on nickel wouldstill have a good surface appearance after 20 mg had worn away. Yet,even after losing 40 mg of weight the Au coated coin still retained itsgolden color and only on the edges of the coin was Au layer abradedaway.

EXAMPLE 5

Eight pure Ni Canadian 25¢ pieces were Au plated using the procedure setforth in Example 1 and annealed at 450° C. for 4 min. The measuredweight of gold electrodeposited was 6 to 8 mg per coin. These coinsalong with 8 Canadian nickel 25¢ pieces and 8 U.S. cupro-nickel 25¢pieces were repeatedly placed by hand through a mechanical vendingmachine coin sorting mechanism manufactured by Coinco, 868 ProgressAve., Scarborough, Ontario, Canada.

After 8,000 cycles, the U.S. cupro-nickel coins had lost much of theirknurled edge. The edge was quite smooth. After 10,000 cycles theCanadian nickel 25¢ piece showed some slight rounding of the edge andthe Au plated and annealed Canadian 25¢ piece still retained theiroriginal golden color even on the edges.

After 18,000 cycles, the knurled edges on the Canadian 25¢ piece startedto shor a slight wear. The Au plated and annealed coins still retainedtheir original golden color on the coin faces after 18,000 cycles andshowed the same slight knurled edge wear as the pure nickel 25¢ pieces.Some loss of golden color occurred on the edges after 18,000 cycles.

After 18,000 cycles, the total weight of Au on the coins was analyzed as5.25 mg (average) indicating only about a 1.7 mg Au loss and 70% Auretained during the test. Eighteen thousand cycles corresponds toapproximately 50 years of wear on a coin at an average use of once perday in a vending machine.

EXAMPLE 6

The same tumbling test as used in Examples 2 and 4 was repeated exceptthat coinage charge consisted of 2 pure nickel Canadian 25¢ pieces, twoU.S. cupro-nickel 25¢ pieces, 1 Canadian nickel 25¢ piece plated with 6mg of Au and annealed as in Example 1 for 4 min at 450° C. in pure H₂,plus 1 Canadian nickel 25¢ piece plated with 12 mg of Au and annealed tohave the same surface colour as the coin having 6 mg of Au afterannealing. The tumbling mill was rotated for 1 day at 60 rpm, 3 days at27 rpm and 1 to 5 days at 45 rpm. The mean weight loss of the coinsafter this treatment for 5 day and 9 day periods is shown below:

    ______________________________________                                                          5 day 9 day                                                                   mg    mg                                                    ______________________________________                                        U.S. 25¢ cupro-nickel                                                                        18.9    32.7                                              Canadian 25¢ pure Ni                                                                         6.2     14.6                                              6 mg Au on Canadian 25¢                                                                      6.9     --                                                12 mg Au on Canadian 25¢                                                                     7.4     15.8                                              ______________________________________                                    

The 6 mg and 12 mg Au coins were indistinguishable in visual appearanceafter the test.

Both retained their original golden colour even on the edges.

EXAMPLE 7

Two samples of pure nickel-base, gold electroplated (0.3 μm) coin stockwere subjected to Auger analysis to determine the nature of the goldlayer in the unannealed and annealed conditions. The annealing wascarried out as set forth in Example 1. Analysis of the unannealed sampleindicated essentially pure gold in the electrodeposited layer with avery sharp boundary to pure nickel of the base. The annealed sampleshowed essentially a 95%Au-5%Ni composition throughout the gold layerwith a somewhat diffuse but relatively sharp boundary to the essentiallypure nickel of the base.

EXAMPLE 8

The following specimens were dipped into a 5 wt.% NaCl solution, whichhad been adjusted to a pH of 4 with nitric acid, once per day, for 31/2days and allowed to air dry at room temperature for the balance of theday.

6. A gold coated 18/8 stainless steel base.

7. A gold coated 75%Cu-25%Ni base.

The gold was plated and the composite structures annealed essentially asdescribed in Example 1. The gold plated stainless steel sample (6)showed evidence of tarnish. The gold plated cupro-nickel sample (7)showed a slight green oxidation product which was easily wiped off butmight remain in crevices in use.

While in accordance with the provisions of the statute, there isillustrated and described herein specific embodiments of the invention.Those skilled in the art will understand that changes may be made in theform of the invention covered by the claims and that certain features ofthe invention may sometimes be used to advantage without a correspondinguse of the other features.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A composite coincomprising a base having at least two opposing surfaces of nickel andbearing on each of said surfaces a layer of about 0.1 μm up to about 2.5μm thick of gold metallurgically bonded to said nickel base, said goldlayer having been electrodeposited on each of said nickel surfaces andhaving present therein an amount of diffused nickel originating fromsaid nickel of said base sufficient to increase the hardness of saidgold layer but insufficient to significantly destroy the distinctivecolor of said gold layer.
 2. A composite coin as in claim 1 comprising acoin or token structure wherein the layer of gold is about 0.1 μm toabout 1.5 μm thick.
 3. A composite coin as in claim 1 wherein said baseis a monolithic base of essentially pure nickel and said gold layerapart from nickel is a layer of essentially pure gold.
 4. A compositecoin as in claim 1 wherein said base is a corrosion resistant nickelalloy.
 5. A composite coin as in claim 1 wherein said base is acomposite material, the outer surfaces being nickel.
 6. A composite coinas in claim 1 wherein said base is a ferromagnetic material suitable forcoinage operated machines.
 7. A composite coin as in claim 1 prepared bya process comprising providing a base having at least two opposingsurfaces of nickel, electrodepositing on said nickel surfaces a layer ofgold at least about 0.1 μm up to about 2.5 μm thick and thereafterinterdiffusing said gold into said nickel to form a metallurgical bondtherebetween and to introduce into said gold layer an amount of nickelfrom said base sufficient to increase the hardness of said gold layerbut insufficient to significantly destroy the distinctive color of saidgold layer, wherein in the annealed condition the gold layer contains upto about 10% Ni.
 8. A composite coin as in claim 7 wherein the goldlayer contains up to about 5% Ni.